Steel Hull?

[Tensen]

Originally Posted by Jim Cate

I think that Tensen is simply trying to refute the myth that steel yachts are by definition stronger, more impact resistant, more unsinkable than any other building medium.

If I haven't said it this well myself, thanks for saying better than I managed to.

Every time hull material discussions come up, many people immediately dismiss anything other than steel as being weaker or more susceptible to holing, based purely on the material, which is just so very wrong.
I'm sure it isn't done out of malice or fanboy-ism - I assume the people who do this are innocently ignorant of all the facts, so I think it's a good idea to make an attempt to share knowledge.

Originally Posted by Jack Verschuur

Tensen, you're probably right in your assessment of a theoretical hull of equal mass and corrected for density, but it is a hull nobody would build.

If the theoretical does not meet practicality, it needs adjustment if to be realized, or it will remain just that, theoretical.

I'm not suggesting anyone should build one. I'm not even claiming that anyone would be able to find one already built if they tried. I'm saying you can't dismiss a hull as weaker based on the material it's made from - you need to know quite a lot more to be able to judge.

Originally Posted by FamilyVan

An advantage of glass or cf boat is the fact that it can be built with less material weight, giving it more reserve buoyancy, greater stiffness, greater speed, great resistance to hogging, sagging, racking etc. The above factors make them strong boats, but not necessarily good boats to run into solid immovable objects with. Glass won't crumple like steel.

You haven't been paying attention. The fact that steel is ductile and FRPs aren't does *not* mean steel hulls are better for running into immovable objects. Given any steel hull in existence I can easily design an (realistic) FRP hull with massively greater survivability than the steel hull.

We aren't likely to ever see an FRP icebreaker, because steel handles abrasion much better, but this thread is about hitting a whale, and it isn't possible to make a call between two hulls based solely on their material.

[micah719]

A good thing the OP only asked about whale impacts, the most common cause of boat sinkings after leaking throughhulls and meteor strikes. One final addition to the hypothetical test I half-jokingly posted. Before running your boat on the reef, set it on fire. How many cruisers can dance on the head of a (frp) pin?

[Jim Cate]

Originally Posted by micah719

A good thing the OP only asked about whale impacts, the most common cause of boat sinkings after leaking throughhulls and meteor strikes. One final addition to the hypothetical test I half-jokingly posted. Before running your boat on the reef, set it on fire. How many cruisers can dance on the head of a (frp) pin?

Micah, you seem bound and determined to draw this discussion into diverse pathways...

But it is interesting (at least to me) that the only boat that we personally have known that succumbed to a fire was... a steelie!

In this case, a small engine room fire that melted several hoses,the through hull valves for which were not accessible due to the fire... and the boat sank.

Further, now delving into mythology, I have never heard of a frp boat sinking due to osmosis blisters, but I have heard of steel boats sinking because of corroded plates.

The point is that by cherry picking specific cases, one can "prove" almost any hypothesis one wants. One can build to any given required strength in most any medium... I think we can all agree to that, can't we? So, in the long run, the choice of materials should not be driven solely by any one parameter. The wise builder will integrate factors like costs of materials, strength and availability of materials, skills of fabricators, ease of repair in the environment proposed for usage, even aesthetics in making the choice. The blind trumpeting of ideological beliefs (steel is strongest...) is not a good position for a NA, be he a pro or an amateur.

Enough... I thank Tensenfor his contributions, and all for the discussion.

And the bloody whales better watch out...

Jim

[Snowpetrel]

Originally Posted by Snowpetrel

Thanks Tensen, makes sense, although I am not sure that the extra Mpa in all cases would compensate for the extra energy required as the metals yeild?



To use this example. Crudely the toughness is the area under the stress strain curve.

So roughly unadjusted for weight the figures for toughness look something like this?

CFRP carbon fibre 2500*1.5/2=1875
GFRP Glass fibre 500*1.8/2=450
Steel 250*16=4000 (very simplified calcs, 16% elongation)
Alloy =3000? in comparsion

To correct for density using something like 1.5 SG for the plastics, 2.7 for alloy and 7.8 for steel would give us.

CFRP 1875*5.2=9750
GRFP 450*5.2=2340
Steel =4000
Alloy 3000*2.9=8700

I guess this does show that a full carbon laminate is pretty tough. Somehow it doesn't seem intuitive and I guess that a huge number of other factors come into play like sheer strength, notch toughness, after rupture, the effects of welds, stringers and bulkheads. The calcs here are laughably crude. You can tell I never went to varsity. I'm not sure if the carbon laminate is balanced, I'm assuming it is otherwise it's not really a fair test.

My guess is that most people aren't going to build a carbon layup at the same weight as a steel hull, so in the real world I'm sticking with my metal boats, but I'll stop sneering at pansy carbon fibre boats

Just out of interest a 3.2 mm steel hull would therefore equate to about a 16mm thick solid FRP hull and about 8.7mm alloy in this example and 4 mm would be more like 20mm of frp, and 11.6mm of alloy, for the same weight.

Feel free to dispute my figures and assumptions.

Cheers

Ben

Decided I'd better check my figures myself, seeing as no more qualified persons have refined them. It seems the carbon strength in the example I posted is for a unidirectional layup, not a fair or valid comparison for impact on a hull, and you would be hard pressed to find the strengths from polyester and E glass that I had used above. Also I had simplified the calculations for the steel and alloy.

Anyway, reworking the calcs based on more realistic figures and a slightly more accurate (but still crude) method for the metals give the following results.

Carbon fibre 1265*1.6/2=1012*5.2=5263 From Here
Glass fibre 300*1.8/2=270*5.2= 1404 From Here
Steel ((250+370)/2)*25=7750*1= 7750 From here
Alloy ((115+270)/2)*16=3080*2.9= 8932 From Here

According to this even allowing for the same mass, the FRP's don't compare with the metals in the toughness stakes. Aluminium comes out as king, but in reality the HAZ zone around any welds in the aluminium would weaken it to below the steels toughness, and normally the plating is much lighter on an alloy boat. I haven't looked at how kevlars would help. I suspect they would really start to increase the toughness of a FRP layup, but it would be hard to estimate this.

Cores are interesting, they might help, but normally the skin thickness's are reduced when using a core, so in the real world a cored boat might be much less tough?

If my figures are right (and I am not confident of this at all!) then building a FRP boat that has the equivalent toughness to a metal boat is not going to be a trivial exercise. and certainly most (if not all) FRP boats out there wouldn't come close to metals toughness?

[Pelagic]

Originally Posted by Jim Cate

...

But it is interesting (at least to me) that the only boat that we personally have known that succumbed to a fire was... a steelie!

In this case, a small engine room fire that melted several hoses,the through hull valves for which were not accessible due to the fire... and the boat sank.
Jim

Jim I agree with your general point, but ironically you touched on one danger where steel material is a clear winner over GRP. .......FIRE!

The boat sank because of failed hoses and plastic thru hulls, whereas GRP hull and deck would have burned more quickly and fed the fire.

Just a point worth consideration in choosing your hull material.

[Snowpetrel]

Originally Posted by Jim Cate

...
The point is that by cherry picking specific cases, one can "prove" almost any hypothesis one wants. One can build to any given required strength in most any medium... I think we can all agree to that, can't we? So, in the long run, the choice of materials should not be driven solely by any one parameter. The wise builder will integrate factors like costs of materials, strength and availability of materials, skills of fabricators, ease of repair in the environment proposed for usage, even aesthetics in making the choice. The blind trumpeting of ideological beliefs (steel is strongest...) is not a good position for a NA, be he a pro or an amateur.

Enough... I thank Tensenfor his contributions, and all for the discussion.

And the bloody whales better watch out...

Jim

+1 Jim, Properly done it seems that there is room on the ocean for yachts made from Wood, Steel, Aluminium, FRP, and Ferrocement. In the real world with prudent seamanship, good design and construction and normal maintenance all are perfectly safe and capable of venturing most places. There has been plenty of examples of each material sailing successfully in Antarctica for example.

For me I have chosen aluminium for my third yacht, having had GRP, then steel. It has it's faults but I like the combination of performance and toughness that it has. I would have been happy with another steel boat if the right one had been available.

I am slightly concerned by the number of problems that modern production boats seem to be having these days, and the number of rescues. But I think it may be more due to the type of sailor that this modern age of GPS, EPIRB and satphone are producing, than a true reflection on the materials that the boats are built of.

PS glad to hear you had a good trip to New Cal, enjoy the warmth... Snow to 150 meters here in Hobart!

[Tensen]

Originally Posted by Snowpetrel

If my figures are right (and I am not confident of this at all!) then building a FRP boat that has the equivalent toughness to a metal boat is not going to be a trivial exercise. and certainly most (if not all) FRP boats out there wouldn't come close to metals toughness?

Some are off - give me a bit of time and I'll post some more typical ones, hopefully tonight.

[Snowpetrel]

Carbon fibre 1265*1.6/2=1012*5.2=5263 From Here
Glass fibre 300*1.8/2=270*5.2= 1404 From Here
Steel ((250+370)/2)*25=7750*1= 7750 From here
Alloy ((115+270)/2)*16=3080*2.9= 8932 From Here

Weathering steel ((380+520)/2)*20=9000 From Here

This beats the aluminium, though I think all of these higher tensile steels are harder to weld, with things like hot cracking and hydrogen embrittlement being issues. They are supposed to rust less, but i've not heard that it helps as much in a high salt environment. But it would be interesting to find out more about how well they might work in a yacht, and what sort of extra cost is involved.

[FamilyVan]

Originally Posted by Tensen

You haven't been paying attention. The fact that steel is ductile and FRPs aren't does *not* mean steel hulls are better for running into immovable objects. Given any steel hull in existence I can easily design an (realistic) FRP hull with massively greater survivability than the steel hull.

We aren't likely to ever see an FRP icebreaker, because steel handles abrasion much better, but this thread is about hitting a whale, and it isn't possible to make a call between two hulls based solely on their material.

Are you sure its me who hasn't been paying attention? My boat is fibreglass, good thick strong fibreglass. Take a look at my avatar pic, that's my fibreglass boat in ice, all winter long- in great lakes ice.

2 or 3 years ago I was involved in a serious collision between a fibreglass 7 meter Willard RIB and a much much larger tug with a cowboy captain. I was stem on to a concrete wall when slammed by the rapidly accelerating tugs stern. The Willard survived the hit. The transom was pushed forward about a foot, engine compartment looked like an accordian, sponsons were shot, some accordianing in the bow. A month later the Willard was back out and working, she never was quite the same but took it like a champ.

I am not discounting the strength of fibreglass, you just want me to be.

I am saying if I was to select a boat solely on the criteria that it had to run into things repeatedly, day in and day out, such as a tug or ice breaker is intended to do, my first pick would be steel.

Could you design a glass boat to do it, probably. Does Beneteau or Hunter design a boat like that, no they don't.

Sent from my XP7700 using Cruisers Sailing Forum mobile app

[Snowpetrel]

Originally Posted by Tensen

Some are off - give me a bit of time and I'll post some more typical ones, hopefully tonight.

Thanks, It will be interesting to get some accurate figures. I guess the only way to be sure is with a test bed and a stress/strain graph, or a real impact test.. Hopefully you have acess to some better info than what my rather random web searches throw up. Cheers

Sent from my GT-P5210 using Cruisers Sailing Forum mobile app

[Tensen]

@SnowPetrel
You are absolutely right, if you don't use UD laminates the picture isn't so one-sided - but FRPs still beat metals.

You've been a bit generous with the metal's properties, and made an error with your calculation: don't add the yield and ultimate strengths (for any material) just use one.

Normally you'd use the yield strength (which is lower), because you don't want a steel vessel to deform permanently after it flexes on waves or dent without running into something (!), but in this case we're talking about an emergency situation, so you'd use the higher ultimate strength.

Steel used is ABS steel, AH 36 is typical, here's a data sheet: Delta Steel, Inc. - Plate
Aluminium is 5083 H116 alloy: http://www.atlassteels.com.au/docume...v_Oct_2013.pdf
E-glass laminates vary widely depending on configuration, here is one (0 and 90 deg, not UD) with 800 MPa tensile strength: https://www.ecfibreglasssupplies.co....dPlastics.aspx

Redoing your calculations with the above figures (if you're going to divide everything by 2 don't bother, we're only interested in relative answers, so your calculation simplifies to: Percent Elongation * Strength / Density )

Steel: 17% * 480 MPa / 7.85 = 10.39
CF: 1.6% * 1265 MPa / 1.6 = 12.65
Al: 10% * 305 MPa / 2.66 = 11.47
E-glass: 1.8% * 800 MPa / 1.9 = 7.58

Carbon fibre still wins, although not by much.

[Snowpetrel]

Originally Posted by Tensen

@SnowPetrel
You are absolutely right, if you don't use UD laminates the picture isn't so one-sided - but FRPs still beat metals.

You've been a bit generous with the metal's properties, and made an error with your calculation: don't add the yield and ultimate strengths (for any material) just use one.

Normally you'd use the yield strength (which is lower), because you don't want a steel vessel to deform permanently after it flexes on waves or dent without running into something (!), but in this case we're talking about an emergency situation, so you'd use the higher ultimate strength.

Steel used is ABS steel, AH 36 is typical, here's a data sheet: Delta Steel, Inc. - Plate
Aluminium is 5083 H116 alloy: http://www.atlassteels.com.au/docume...v_Oct_2013.pdf
E-glass laminates vary widely depending on configuration, here is one (0 and 90 deg, not UD) with 800 MPa tensile strength: https://www.ecfibreglasssupplies.co....dPlastics.aspx

Redoing your calculations with the above figures (if you're going to divide everything by 2 don't bother, we're only interested in relative answers, so your calculation simplifies to: Percent Elongation * Strength / Density )

Steel: 17% * 480 MPa / 7.85 = 10.39
CF: 1.6% * 1265 MPa / 1.6 = 12.65
Al: 10% * 305 MPa / 2.66 = 11.47
E-glass: 1.8% * 800 MPa / 1.9 = 7.58

Carbon fibre still wins, although not by much.

Hi Tensen, thanks for the links, all good data. That 800 mpa GRP is pretty wild, sure it's not unidirectional?

On your calcs, I understand toughness as the area under the stress strain curve, so assuming things run linear up till yeild on the metals, and then plateau till max strength before breaking at a lower figure we can approximate the area as the average of the yeild MPA and max MPA, times the elongation. as a kind of trapezoid shape, as opposed to the triangular shape of the FRP's



so for this example the area is roughly
((250+425)/2)*25% or 84.375

The FRP's shape is more like a triangle, so the area under them is (half MPA*elongation%) so for CF it is 1265*1.6%/2=10.12



So to use my method on your figures we get these sort of numbers...

Steel: 17% * 480 MPa / 7.85 = 10.39 17%*0.5(250 Mpa+480 Mpa)/7.85=7.9
CF: 1.6% * 1265 MPa / 1.6 = 12.65 1.6%*1265 Mpa*0.5/1.6=6.325
Al: 10% * 305 MPa 66 = 11.47 10%*0.5(115 Mpa+305 Mpa)/2.66=7.89
E-glass: 1.8% * MPa / 1.9 = 7.58 1.8%*800 Mpa*0.5/1.9=3.79

My calcs in red. This shows a the metals edging ahead of the carbon on a weight specific toughness. And I think the elongation values for the metals are pretty conservative, here gives it by law as 20% minimum for A36 steel, and other sources show higher values. Same with the aluminium, the 10% is close to the minimum requirements.

If we use the more typical or minimum values for the FRP's rather than the maximum values as used here we substantially reduce their toughness as shown. Given the quality control issues with laying up large FRP structures, it's going to be very hard to insure maximum strengths over the whole structure, look at some of the issues around resin infusion, and dry laminates for example.

So on balance I am pretty happy saying that the metals are tougher than FRP on a weight basis, though I agree that the advanced FRP laminates in many cases wins the weight specific strength and stiffness battles, without even using cores, hence their use on most modern racing boats, and I am very surprised how close the carbon is getting to the metals. Kevlar or spectra embedded into the matrix to hold it all together after failure is an interesting idea that seems to be working in the concrete reinforcement field. I do recall something about them trialling a dyneema epoxy FRP layer on steel oil tankers to reduce the chances of rupture after grounding, before they settled on the Double hull concept (for all it's faults).

The failure modes definitely are much more complex in the real world than these numbers show. I am sure the FRP's don't fail catastrophically in sheet form as the tensile stress strain graphs show. Sheer strength and notch toughness would also be interesting factors as well. It's interesting that bulletproof vests are made from kevlar and dyneema, and Hi tech tank armour uses brittle ceramics and steels.

Cheers

Ben

[Tensen]

[QUOTE=Snowpetrel;1892718]

My calcs in red. This shows a the metals edging ahead of the carbon on a weight specific toughness.

My apologies to you, I assumed you were just making an error rather than taking a more accurate shape.

Yes, I agree with your figures, and the result is undeniable. I will have to stick only to my original argument - FRPs are stronger by weight

And I think the elongation values for the metals are pretty conservative, here gives it by law as 20% minimum for A36 steel, and other sources show higher values. Same with the aluminium, the 10% is close to the minimum requirements.

A36 is just an example of steel that meets ABS requirements, but law or no law we can see that some manufacturers do only guarantee 17%. Some give 19%, and some 20%. The aluminium may also vary between manufacturers, but only by a percentage point or two.

If we use the more typical or minimum values for the FRP's rather than the maximum values as used here we substantially reduce their toughness as shown.

You have to ensure you're using the correct weave and fibre percentage. There's no point listing CSM strengths or low fibre percentage strengths for example, when no-one worried about strength would use them.

Sheer strength and notch toughness would also be interesting factors as well. It's interesting that bulletproof vests are made from kevlar and dyneema, and Hi tech tank armour uses brittle ceramics and steels.

Sheer strength is far less important, most failures don't involve it. In a cored hull the core is responsible for sheer strength.
Ceramics behave quite similarly to high performance composites, but when you start looking at armour hardness becomes very important.

To get back to the specifics of the thread OP: a whale strike is presumably going to be on the forefoot, which means the composites involved will be almost entirely unidirectional: the vast bulk of them will be in the major stringer running along the keel and up the stem.

You are quite right about specific toughness in general, but in this particular case the circumstances mean FRPs will use UD strength, and so have higher toughness.

[Snowpetrel]

Thanks Tensen for taking time to review and comment on the figures I posted. I have learnt a lot about materials that my mechanical engineering and drafting diploma didn't really teach (or rather they tried to but it didn't sink in to my thick skull!)...

I found this link which is interesting. It gives a stress/strain graph for GRP panels. Shows an elongation around 6% and a slightly non linear shape. The Mpa tensile strength is low, but it's toughness is comparable to your 800Mpa super sample. Interesting, given I thought polyester resin only had a maximum elongation of 1.5-2%? Maybe just the glass is doing the work, or the slowly failing matrix was creeping? I think the load was applied very slowly, weird...

I can see lots of room for improving the impact toughness of the average GRP yacht. Eg the HC offshore explorer has a inner bow section separated by a crash zone of 6 inches of foam. This plus the kevlar would probably give it a better impact survival chance than a normal steel hull. It's a shame kevlar has such appalling compressive strength, otherwise it would be perfect for hulls.

The big question is if it is actually a real problem, modern materials are so tough anyway and proper cruising boats are built strong enough that I don't think it's likely that they would be damaged in any normal collision with a whale or object, and I think the real world frequency of boats being holed at sea shows this. Ie It is not common at all, and normally any damage is to appendages not the panels

The whale we hit (probably a juvenile sperm whale) caused no apparent damage. We hit it while surfing down a wave at around 15 knots in a 30 knot southerly (with the kite up) about 200 miles offshore from Sydney. The boat was a lightly built plywood spencer 45 (smaller version of ragamuffin) Arguably plywood is one of the least tough materials (but very strong and stiff), and still no problems.

The risk of whale attack seems real, and scary, but very rare, and it would seem that a isotropic material with reasonable toughness like GRP or Steel can resist the impact in most cases.

Cheers

Ben

[RaymondR]

From a toughness viewpoint the graphs above pretty well say it all. The composites are brittle materials even though they may have higher tensile strengths. A point loading impact will lead to a rapid, progressing failure of the composite materials where the metals will yield then deform and absorb the energy of the impact.